1. Field of the Invention
This invention is concerned with towing arrays of geophysical devices in a body of water behind a specially-designed ship for the purpose of conducting three-dimensional geophysical surveys such as might be found in classes 367 and 181/0.5.
2. Discussion of the Prior Art
In seismic surveys at sea, a ship tows a suitable sound source such as an air gun (water guns, gas guns or sparkers may also be used). At intervals of perhaps eight or ten seconds, the sound source is fired to produce an acoustic wave having a spectrum in the 10-100 Hz (cycles per second) range. The ship also tows an elongated array of hydrophones. The acoustic waves generated by the sound source propagate through the water, into the earth and are reflected by subbottom earth layers. The reflected acoustic waves then return to the surface to be detected by the hydrophones. The hydrophones convert the acoustic waves to electrical signals which are sent to a signal utilization device for processing and archival storage. From the processed signals, geophysicists produce maps of the respective earth layers.
The energy output of a single sound source of the type hereinbefore listed is relatively weak. In addition, certain types of sound sources create objectionable wave forms such as bubble pulses or are deficient in certain frequency bands within the seismic spectrum. Therefore, the seismic sound sources are grouped in two- or three-dimensional tuned arrays, having a desired geometrical pattern, containing a large number of sources to produce a more robust seismic spectrum.
The array dimensions parallel to the ship's track may be set up easily enough by simply adjusting the lengths of the tow lines to the respective guns of the array. Perpendicular to the track, the lateral dimensions are sometimes established by use of paravanes such as shown by U.S. Pat. Nos. 4,130,078 and 4,525,813. That arrangement is not satisfactory because the locations of towed paravanes are never known accurately. U.S. Pat. No. 4,038,630 teaches use of fixed booms or outriggers mounted on each side of the stern of the ship to increase the lateral extent of the array. A somewhat more elaborate configuration is illustrated in U.S. Pat. No. 4,480,574. Here, several seismic sound sources are towed from pivotable booms on each side of the ship. Each sound source is manipulated by an individual winch, there being several winches distributed along each boom. That teaching is not very practical because first, the winch mechanisms are directly exposed to sea water, subjecting the winches to corrosion problems. Secondly because the winches and cables are quite heavy, the number of winches that can be safely mounted on booms of reasonable size must be limited to avoid structural failure of the booms. It is to be observed that the '630 and the '574 patents teach deployment of geophysical devices of but a single genus, namely air guns.
In three-dimensional (3D) seismic exploration, the geophysical ship proceeds along lines of survey separated by some distance such as 80 meters. A typical hydrophone array may be 3000 meters or more long and may include 1000 or more hydrophones, all encased in a plastic jacket. Such a hydrophone array is commonly referred to as a streamer cable. If a single streamer cable is towed along a line of survey and insonified by an array of sound sources, reflection data from a single line of profile are acquired. If two or more streamer cables, towed from fixed points on the ship, laterally separated by the desired survey-line spacing, are towed behind the ship, then two or more lines of profile can be surveyed at one pass of the ship, a decided economy of ship's time and charges.
The beam of a typical geophysical survey ship rarely exceeds 10 to 15 meters. If two streamer cables are towed directly behind each side of the ship, the cable separation is inadequate for dual-line surveying. Attempts have been made to increase the lateral separation of the cables by use of paravanes. But because of the towing drag of a streamer cable, some 3000 to 6000 pounds (as compared with 100-200 pounds drag force for sound sources), paravanes of manageable size cannot achieve the necessary lateral "lift" to hold the cables in position. Furthermore, it has been found that some ships are unable to develop enough power to tow the paravanes at an acceptable velocity for economical survey work.
A long streamer cable under tow in the water is subject to erratic cross currents. The precise position of the cable is unknown. U.S. Pat. No. 4,376,301 teaches use of transponders or pingers submerged below the water surface, outboard of the ship, separated by 20 to 40 meters. Sound pulses radiated by the pingers are detected by special receivers secured along the streamer cable at selected intervals. The travel time between the pingers and the respective receivers is measured to calculate the receiver positions relative to the ship by triangulation. Because of the relatively short base line, 20-40 meters, compared to the great length of the streamer cable, at least 3.0 km, that method is not very accurate. That problem is excaberated in view of errors in the ship's heading which introduce a skew error in the reference base line between the pingers. Heading errors are caused by the unknown gyro-compass precession rate between in-port calibrations. Therefore the absolute location of the streamer cable in two-dimensional space is indeterminate.
Another cable-location technique is taught by U.S. Pat. No. 4,532,617. That method involves ranging on the streamer cable from two different locations involving use of a second slave ship. The cost of the additional ship makes that practice, as taught by the '617 patent, unattractive.
At least one geophysical contractor advertises use of a tail buoy secured to the far end of the streamer cable. The tail buoy is equipped with a navigational device and the head end of the streamer cable is held in place by paravanes. Although the location of the tail of the streamer cable may be known, the location of the head end is not accurately known because of the paravane problem mentioned supra. That method is impractical.